Astronomy Picture of the Day

Explanation: Spiral galaxies viewed face-on display a grand design, with graceful spiral arms traced by bright star clusters and glowing stellar nurseries. When seen edge-on, their appearance is very different but no less striking as their central regions bulge and dark cosmic dust lanes appear silhouetted against starlight from flattened galactic disks. This masterful mosaic of digital images shows nine prominent edge-on spirals arranged as follows: top; NGC2683, M104, NGC4565, middle; NGC891, NGC4631, NGC3628, and bottom; NGC5746, NGC5907, and NGC4217. Perhaps the best known of these is M104 (NGC4594) whose more descriptive moniker is the Sombrero Galaxy. Notably, the edge-on perspective of these galaxies allows a measurement of their galactic rotation speed using the Doppler effect. Plotting rotation speed versus distance from the center determines a galaxy's gravitational mass and historically led to premier evidence of the mysterious dark matter.